When a wafer is processed in a semiconductor manufacturing system, the orientation of the wafer is typically aligned by an alignment mechanism in advance on the basis of a notch (V-shaped slit indicating a reference direction corresponding to the direction of a crystal array of the wafer). The alignment mechanism mounts the wafer on its rotatable stage, and rotates the wafer by 360° or more, and then detects the direction of the wafer on the basis of obtained data of the peripheral edge of the wafer. In this case, the alignment mechanism usually adopts a method that pushes the wafer at, for example, three points on the periphery of the wafer to center the wafer; or a method that calculates the positional deviation of the center of the wafer on the basis of the profile data of the wafer periphery and causes a transfer arm to receive the wafer such that the positional deviation is corrected.
A so-called multi-chamber system in which a plurality of process chambers are connected to a transfer chamber is also equipped with an alignment mechanism. When a wafer is transferred from the alignment mechanism to the process chamber, there is a possibility that, for example, a malfunction of a transfer arm will cause a positional deviation of the wafer. In addition, when a processed wafer is removed from an electrostatic chuck, there is also a possibility that a residual electric charge may cause excessive force to act on the wafer to displace the wafer. If the wafer displaces from its proper position on the transfer arm, when the wafer passes through a transfer port of a process chamber and that of a load lock chamber, there is a possibility that the wafer will collide with the chamber wall. Moreover, in the case of a wafer before processed, even if the wafer is carried into the process chamber, the wafer is not correctly mounted on a mounting table. Therefore, it is not possible to ensure the in-plane uniformity of the process. For this reason, in the multi-chamber system, the position of a wafer mounted on the transfer arm is detected by use of wafer detection sensors.
With the use of such detection sensors, in one example, the detection of the wafer position is performed by stopping a transfer arm, and then detecting the peripheral edge of the wafer by three sensors and calculating the center position of the wafer on the basis of the result of the detection. In another example, the detection of the wafer position is performed by moving a transfer arm so that the wafer crosses detection areas of two sensors, and calculating the center position of the wafer on the basis of each encoder value obtained when the wafer crosses the detection areas of the two sensors.
Incidentally, in order to quickly detect the positional deviation of a wafer mounted on a transfer arm, it is desirable to provide a system with a plurality of monitoring (position-detecting) areas. However, since the foregoing sensors configured to detect the wafer position when the transfer arm is stopped is expensive, the provision of plural monitoring areas leads to increase in the total cost of the system. In addition, in a case where plural sensors are provided in one monitoring area and plural monitoring areas are provided in a system, the layout of the sensors becomes difficult. Moreover, if the number of sensors is large, it is possible that the positional relationship between the sensors is not accurate. Inaccurate positional relationship between the sensors will result in an error in calculating the center position of the wafer. JP01-48443A (page 18, upper part; FIG. 2 to 5) discloses a technique for detecting a deviation in the position of a wafer while a transfer arm is moved. However, this technique cannot solve the foregoing problems.